1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly to a direct-view-type LCD device capable of achieving a higher contrast ratio.
2. Description of the Related Art
LCD devices have the advantage of realizing a higher definition with a lower power dissipation and are used for a wide range of applications from a small-screen cellular phone to a large-screen television monitor. However, there is a defect in the LCD device that the contrast ratio of the LCD panel alone in a dark environment is lower than that of a CRT, that (3000:1) of a plasma display panel, which are also used as a television monitor similarly to the LCD panel, and that of a field-emission display panel called FED/SED, and is at most on the order of 1000:1. Therefore, there is pointed out the problem of insufficient feeling of live performance during representing image sources such as a motion picture having a higher power of expression especially in the dark area.
In order to solve the above problem, there has been developed a technology for controlling the light intensity of the backlight according to the image to be displayed, thereby improving the contrast ratio on the display screen, with the contrast ratio of the LCD panel itself being left intact. In a conventional backlight unit having a surface-emission light source, however, a cold-cathode tube having a narrow dynamic range is used as the light source. Thus, improvement of the contrast ratio by controlling the light intensity of the backlight according to the image to be displayed is limited to around 2000 to 3000:1.
It is to be noted that the cold-cathode tube of the backlight unit has a shape of rod. Thus, if there are a high luminance area and a low luminance area concurrently represented on the same screen of the LCD device, the luminance of the backlight cannot be regulated area by area, resulting in a poor improvement of the contrast ratio obtained by the luminance control of the backlight. Therefore, if the image represented on the screen has a higher luminance area and yet is desired to emphasize the reproducibility in the lower luminance area, the effective contrast ratio is lowered due to the presence of the higher luminance area.
In order to solve the above problems, the contrast ratio of the LCD panel should be drastically improved. However, as described before, the contrast ratio of the LCD panel alone is at most about 1000:1. Techniques for manufacturing LCD devices capable of remarkably improving the contrast ratio thereof without improving the contrast ratio of the LCD panel itself are described, for example, in Patent Publications JP-1989-10223A and JUM-1984-189625A. In these technologies, a multi-panel LCD structure wherein two or more LCD panels are stacked one on another is employed in a LCD device to reduce the black luminance, i.e., luminance upon display of dark image, thereby improving the total contrast ratio of the LCD device. JP-1989-10223A describes a multi-panel LCD device that achieves a contrast ratio exceeding the contrast ratio of a LCD device having a single LCD panel, which fact is confirmed by measuring the overall contrast ratio of the LCD device by using laser. It is described therein that two-panel LCD device achieves improvement of contrast ratio up to about 100:1 by using LCD panels having a contrast ratio of about 10 to 15:1, and that three-panel LCD device achieves a contrast ratio of 1000:1.
The technique of the multi-panel LCD device is also described in Patent Publications JP-2004-512564A and JP-2001-201764A. The technique described in JP-2004-512564A does not relate to a LCD device realizing a higher contrast ratio, and related to the technique of automatic stereoscopic image display. The technique described in JP-2001-201764A does not relate to a LCD device realizing a higher contrast ratio, and relates to the technique of uniquely designed LCD device by using the multi-panel LCD device.
For driving a projection LCD device having the multi-panel LCD structure, a common signal can be used to drive all the LCD panels without involving any problem, because these LCD panels pass the light substantially perpendicular to the LCD panels to project an image on a screen. However, such a common signal involves a problem in an ordinary direct-view LCD device having the multi-panel LCD structure, wherein a light source emitting a scattering light is used for image display as in the case of the LCD device using a backlight unit. The problem is that the distance or gap between adjacent LCD panels generates a parallax depending on the viewing angle of an observer, the parallax preventing the light passed by a pixel of the rear-side (or light-receiving-side) LCD panel from passing through the corresponding pixel of the front-side (or light-emitting-side) LCD panel. If the observer observes the display screen in a slanted viewing direction, the parallax deviates the direction of the pixel of the front-side LCD panel from the corresponding pixel of the rear-side LCD panel, and thus an edge of the image in particular, across which the brightness of image is significantly changed in general, will be observed as double lines. Thus, the observer feels a sense of discomfort.
FIGS. 23A, 23B, and 23C schematically show the image of pixels of the multi-panel LCD structure located at different positions of the display screen, as observed from the front center of the display screen. The multi-panel LCD structure includes two LCD panels in this case. FIG. 23A shows the image of a pixel located on the observers' left of the display screen, FIG. 23B shows another pixel located at the center of the display screen, and FIG. 23C shows the image of another pixel located on the observers' right of the display screen.
In the case shown in FIG. 23B, the pixel of the front-side LCD panel and the pixel of the rear-side LCD panel are observed to exactly overlap each other, without causing any problem. However, as shown in FIGS. 23A and 23C, the pixel of the front-side LCD panel and the pixel of the rear-side LCD panel are observed not to overlap each other, causing the problem of reduction in the luminance. In addition, the deviation of the pixels generates a bright area and a dark area which are arranged periodically, to generate an interference pattern such as a moiré. Thus, the image quality of the LCD device is degraded. The dark area is generally formed by interconnect line or black matrix (hereinafter, simply referred to as black matrix) of the LCD panel which blocks the light.